Clearance compensating mechanism, especially for valve drives of internal combustion engines

ABSTRACT

A clearance compensating mechanism in reciprocating driving linkages, especially for valve drives of internal combustion engines. The mechanism is provided with a piston in front of an oil-filled pressure chamber, which piston moves into a cylinder in a downward direction, thus leaving an annular gap so as to permit a passage for minor oil leakage. The pressure chamber is in communication with a reservoir, which also contains oil, by means of a passage having a relief valve with a valve body held by, for example, a cage and adjoining the pressure chamber below an inlet point in the annular gap. A compression spring can be provided for pretensioning the piston along the lines of its movement out of the cylinder.

United States Patent Guido [451 June 20, 1972 s41 CLEARANCE COMPENSATING 2,665,670 1/1954 Adkins ..|23/90.59 MECHANISM, ESPECIALLY FOR 2,676,579 4/1954 Gemer... 123/9063 X VALVE DRIVES 3'353'332 $11322 Tf"" iiiti s an er COMBUSTION ENG 2,795,218 6/1957 Heiss ..l23/90.55 [72] Inventor: Jurgen Guido, Borsigstrasse 4, Regen- 2,954,015 9/1960 Line .123/90.59 X sburg-Neutraubling, Germany 3,025,842 3/ 1962 Van Slooten 123/9058 [22] Filed: March 1970 Primary Examiner-Al Lawrence Smith [21] Appl. No.: 18,090 A!t0rneyCraig, Antonelli & Hill 57 ABSTRACT [30] Foreign Application Priority Data 1 A clearance compensating mechanism in reciprocating driv- Match 1969 Germany l9 l2 046'8 ing linkages, especially for valve drives of internal combustion engines. The mechanism is provided with a piston in front of [52] US. Cl ..l23/9:).l57, 123910.586, 123/1906: an oibfilkd pressure chamber, which piston moves into a [5 l 1 Cl g g g? 2 cylinder in a downward direction, thus leaving an annular gap [58] he l 5 6 so as to permit a passage for minor oil leakage. The pressure chamber is in communication with a reservoir, which also contains oil, by means of a passage having a relief valve with a [56] Referenm Cited valve body held by, for example, a cage and adjoining the UNITED STATES PATENTS pressure chamber below an inlet point in the annu lar gap. A compression spring can be provided for pretensionmg the 2,158,222 511939 g y: 5 5- piston along the lines of its movement out of the cylinder. 2,308,858 1 1943 urk ar t 2,438,631 3/1948 Bergmann ..123/90.59 8 Claim, 3 Drawing Figures PATENTEDJUHZO I972 3.670.707

/5 Z Z4 Z0 CLEARANCE COMPENSA'I'ING MECHANISM, ESPECIALLY FOR VALVE DRIVES OF INTERNAL COMBUSTION ENGINES BACKGROUND OF THE INVENTION The present invention relates to a clearance compensating mechanism in reciprocating drive linkages, especially for valve drives of internal combustion engines, and more particularly, to a clearance compensating mechanism with a piston in front of a pressure chamber filled with a liquid mechanism with a piston in front of a pressure chamber filled with a liquid medium such as oil, which piston or plunger plunges into a cylinder in the downward direction, thereby leaving an annular gap or slot and permitting a minor medium or oil leakage passage. This pressure chamber is in communication with a reservoir or storage chamber, which likewise contains oil, by means of a passage having a relief valve with a valve body held by a cage or the like and adjoining the pressure chamber below an inlet point in the annular gap. A compression spring is provided and pretensions the piston along the lines of its movement out of the cylinder.

in clearance compensating mechanisms, it is important that the pressure chamber, which has its largest volume at the beginning of the valve opening phase, maintains this volume as much as possible during the open period of the valve, so that the flow losses or leakage occurring during the passage of medium through the valve remains at a minimum. However, it was found in the practical application of conventional compensating units that considerable difficulties are encountered in keeping the oil present in the pressure chamber and passing therethrough free of air bubbles to such an extent that the desired incompressibility of the pressure chamber is retained. As experience has shown, air bubbles are produced in the pressure chamber of the compensating unit at the moment when the relief valve of the unit is opened during the movement of the piston out of the cylinder, and oil enters or is even sucked from the reservoir through the passage into the pressure chamber, which is pressureless at this instant. It has been found from experience that air bubbles which are at first extremely small separate from the oil at this moment, even in case of a so-called closed oil cycle of the unit, which cycle is thus, for example, independent of the oil circulation of an internal combustion engine. These air bubbles, upon an expansion of the oil, pass over into the pressure chamber and combine into increasingly larger air bubbles during the time the medium flows through the pressure chamber, unless steps are taken to permit the air bubbles to escape sufficiently rapidly.

Numerous measures have become known which attempt to effect an appropriate ventilation of the pressure chamber of the clearance compensating unit. According to US. Pat. No. 2,145,484, the cylinder of the compensating device forms simultaneously the tappet or push rod of a valve drive, and has lateral vents at the uppermost place in the zone of the reservoir as well as in the zone of the pressure chamber, respecitvely. Through these vents, the air separated from the oil is to escape when the valve is in its open position. Similar ventilating devices are provided in accordance with U.S. Pat. No.- 2,246,330, German Pat. No. 902,198 and according to German Utility Model 1,983,334.

Yet, in all of the aforementioned known compensating devices, an undesired compressibility of the pressure chamber was found, surprisingly, to exist during operation, which compressibility takes place during the opening stroke of the valve mechanism and heretofore seemed to be unexplainable. Although the above-mentioned conventional compensating devices all operate with a so-called open oil cycle or circulation, i.e. wherein lubricating oil is fed to the device from the oil circulation of an internal combustion engine and after escape from the pressure chamber is discharged along the annular gap between the piston and the cylinder and back into the oil sump of the oil cycle of the internal combustion engine, the same unfavorable experience was also found to occur even in connection with so-called closed oil cycles of the clearance compensating devices wherein the reservoir is in the piston and any escape of oil from the device is prevented by means of a seal provided at the outer end of the piston. Attempts at preventing any subsequent formation of air bubbles in the pressure chamber by a one-time introduction of an especially well ventilated oil were in vain. Thus, in closed cycles, as compared to the open cycles, the only advantage practically remaining is that any interference with the flawless operation of the compensating device by lubricating oil contaminated by the operation of the internal combustion engine is excluded even without the provision of special filter inserts.

SUMMARY OF THE INVENTION It is one of the aims of the present invention to avoid the above-described deficiencies and to perfect the clearance compensating mechanisms set forth above, independently of whether they operate with a closed or an open oil circulation, in such a manner that the pressure chamber of the mechanism cannot any more be subjected to a disadvantageous compression during the pressure load on the linkage and thus, for example, during the valve opening period of an internal combustion engine provided with the mechanism.

The underlying problems are solved in accordance with the present invention by providing in the aforementioned clearance compensating mechanism that the wall surfaces defining the pressure chamber with the cage or the like on the sides and at the top, all extend, at most, somewhat obliquely with respect to the leakage oil discharge direction toward the annular gap in the upward direction, or toward the highest inlet point of this gap. It is further provided that the pressure chamber joins the annular gap or the highest inlet point thereof at least approximately up to the greatest throughflow cross section between the passage and the annular gap, or to the highest inlet point thereof, with a throughglow cross section which increases constantly toward the passage in an ap proximately funnel-like fashion.

Thorough investigations of clearance compensating mechanisms constructed in the above manner have shown, unexpectedly, that the air bubbles formed in the pressure chamber during the influx of the oil flow into the annular gap, together with the oil leakage escaping into the annular gap, solely due to the buoyant forces effective thereon and the flow forces exerted by the leak oil on the bubbles. This reliable efflux of the air bubbles, which can readily be proven in compensating mechanisms fashioned according to the present invention by means of cylinders which are made transparent for purposes of the experiment, can only be explained by the fact that, in the path of motion of the air bubbles with a constantly decreasing throughflow cross section, the flow velocity of the oil also increases constantly. Thus, the deformation energy required during the deformation of the air bubbles, which are at first more or less spherical, into more or less flattened circular ring segments, as seen in the flow direction, can be provided by the correspondingly increasing flow energy of the leak oil flowing into the annular gap.

In contrast thereto, if there are any obstacles in the throughflow path of the air bubbles through the pressure chamber, in the shape of sharpedged angular deflections of the flow path, or sudden changes in cross section, as is the case in the conventional devices mentioned in the foregoing, then the air bubbles which are present are deflected laterally out of the flow path of the oil leakage into the existent zones of lower flow velocity. Consequently, the buoyant force of the air bubbles, which is minor anyway as compared to the oil, is not any more sufficient for returning the bubbles into the main flow path of the oil leakage.

According to the preferred constructional embodiment of the present invention, the bottom surface of the piston plunging downwardly into the cylinder in a conventional manner is, at least approximately, a conical surface having a cone angle of at most about 120 and preferably only about In accordance with a further preferred embodiment of the present invention, the bottom surface of the piston likewise plunges downwardly into the cylinder and passes over along its outer rim into the cylindrical side wall of the piston with a phase portion having a cone angle of between about 20 and 40. Such a phase portion, which according to other developments of the present invention, preferably forms a cone angle of about 30 and has a radial width of about 1 mm., surprisingly contributes particularly advantageously to the effect that the air bubbles in conjunction with the flow energy of the oil entraining these bubbles are sufficiently flattened and are carried along into the annular gap.

Other embodiments described hereinbelow relate to further constructional details of the clearance compensating mechanism of the present invention.

BRIEF DESCRIPTION OF THE DRAWING These and further objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawing which shows, for purposes of illustration only, several embodiments in accordance with the present invention and wherein:

FIG. I is an axial sectional view of an embodiment of a clearance compensating mechanism according to the present invention wherein a piston of the mechanism plunges into an associated cylinder in the downward direction;

FIG. 2 is an enlarged fragmentary detail of Fig. 1;

FIG. 3 is a cross-sectional view of the mechanism taken along line IIIIII of FIG. 1.

DETAILED DESCRIPTION OF THE DRAWING Referring now to the drawings and, in particular, to FIG. 1, the clearance compensating mechanism designated generally by the numeral I is provided at the upper end of a valve push rod 2 of an internal combustion engine which otherwise is not illustrated. The compensating mechanism consists essentially of a cylinder 4 attached to the valve push rod 2 at its lower closed end 3, a piston 6 extending downwardly into this cylinder 4, thereby leaving an annular gap of about 0.02 mm. and permitting the passage of a certain amount of oil leakage. A pressure chamber 7 is formed underneath the piston 6 in the cylinder 4, and a reservoir or storage tank 8 is disposed in the piston 6 for additional oil in addition to being in communication with the pressure chamber 7 by way of an axial passage 9 and with an external annular groove 10 of the piston 6 provided at the outer end of the annular gap 5 by way of radial bores 11 of the piston 6.

At its outer end extending from the cylinder 4, the piston 6 carries a head 13 sealing off the reservoir 8 toward the outside and forming a cup 12 for a conventional rocker arm (not shown) of the valve control mechanism. This head 13 is sealed with respect to the cylinder 4 by a gasket or sealing ring 14. Furthermore, the piston 6 is biased toward the outside by a coil compression spring 15 provided in the pressure chamber 7. In FIG. I the spring is shown in its most relaxed position and the piston rests on or against a spring ring 16 inserted in the cylinder 4 close to the outer end thereof in a corresponding internal annular groove. On the side of the pressure chamber 7, the passage 9 is normally sealed off by the ball 17 of a relief valve.

In the above-described components, the illustrated clearance compensating mechanism basically corresponds to a known construction with a closed" oil circulation, which is thus independent of an external oil source constituted, for example, by the lubricating oil cycle of an internal combustion engine. Whereas in other clearance compensating devices 70 with an open oil circulation, the oil present in the pressure chamber is continuously replenished by oil from the lubricating oil cycle of an internal combustion engine or the like, and disturbances can easily occur when contaminated oil flows measures necessary in some cases, the closed oil circulation offers the advantage that, with the originally supplied and contained oil being pure, no subsequent troubles need be anticipated by oil contarninations.

As can be further seen from FIG. I, the bottom surface 18 of the piston 6, following the axial passage 9, forms a conical surface having a cone angle a of about 90". Also, the bottom surface 18 continues, alone its outer rim, into the cylindrical side wall of the piston 6 with a phase portion 19 having a cone angle )3 of approximately 30.

As seen in FIG. 2, in particular, the coil compression spring 15 rests at the bottom surface 18 beside the inner rim of phase portion 19 on a number of pins 20 which extend approximately radially in the outward direction, and which are oriented toward the outside with respect to the external end of the cylinder 4. In this connection, the coil compression spring 15 rests with its outermost winding 21 on or against the pins 20 at a spacing 22 of about 1 mm. While customary phase portions of comparative components are provided at faced parts by means of a lathe operation and normally have a radial width of only about 0.5 mm, the radial width 23 of phase portion 19 is 1 mm. The ball 17 of the relief valve is held in its locking position on the lower aperture of the passage 9 by means of a narrow leaf spring arm 24, which extends radially underneath the ball from only one side.

The aforementioned clearance compensating mechanism in accordance with the present invention operates as follows: With the valve closed and the valve push rod 2 accordingly relieved, the piston 6 is in the outer terminal position as indicated in FIG. 1. The pressure chamber 7 is filled with oil, of which it can be assumed from practical experience, that it contains extremely fine air bubbles. Due to the buoyant force effective on these bubbles, they tend to rise upwardly and combine into larger air bubbles, while simultaneously moving toward the outside along the conical bottom surface 18 of the piston 6.

During the subsequent opening stroke of the valve, the piston 6 is stressed toward the valve push rod 2 with a force which is substantially larger than the force of the coil compression spring 15. This first-mentioned force must be absorbed substantially by the oil present in the pressure chamber 7. Since the total length of the valve mechanism consisting of the valve push rod 2 and the clearance compensating device 1 with the valve in the closed position, is somewhat longer than corresponds to the constructional rated length, the annular gap 5 between the piston 6 and the cylinder 4 of the clearance compensating mechanism 1 is dimensioned so that, with the valve being open and the pressure chamber 7 being under a corresponding load, a minor amount of oil leakage can be discharged through the annular gap 5, the annular groove 10, and the bores 11 into the reservoir 8, so that the valve mechanism, at the instant of reclosing of the valve, is shortened at least to such an extent that a flawless seating of the valve is ensured. This means that the valve mechanism, at the instant of seating of the valve, must always be a little shorter than corresponds to the above-mentioned rated length, for safety reasons. Accordingly, during the open period of the valve, a small amount of leak oil flows in each instance through the annular gap 5 into the reservoir 8. Above the oil level 25 of this reservoir, there is always a certain air cushion 26.

In the illustrated clearance compensating mechanism, the conical bottom surface 18, the above-described supporting of the coil compression spring 15 on the pins 20 and, in particular, the aforementioned phase portion 19 along the outer rim of the bottom surface 18, have the effect that, in the path of motion of the air bubbles with a constantly decreasing throughflow cross section, the flow velocity of the oil also increases constantly. Consequently, the deformation work required in the deformation of the air bubbles 27, which at first are more or less spherical, in the flow direction into more or less flattened circular ring segments can be provided by the through the annular gap 5, thereby making special filtering correspondingly increasing flow energy of the oil leakage flowing off into the annular gap. The air bubbles produced during the relieving of the pressure chamber 7 are removed in this manner reliably and quickly into the annular gap 5 and into the reservoir 8 via the annular groove 10 and the bores 11, so that no troublesome compressibility is possible any more in the pressure chamber.

The present invention, quite apparently, is not-bound to all details of the above-described embodiments. It is unnecessary, in particular, to provide the compression spring 15, which biases the piston 6 of the clearance compensating mechanism toward the outside, in the pressure chamber 7 proper. Rather, it is also possible to arrange the compression spring around the head 13 of the piston 6 extending out of the cylinder 4 and support the spring externally on an outer collar of the head 13 and internally on the end of the cylinder 4 disposed therebeneath. Even though a cone angle a of the bottom surface 18 of the piston 6 according to FIG. 1 of about 90 is preferred, this angle could also be larger, up to about 120, or it could also be smaller than 90". Furthermore, the cone angle ,8 of the phase portion could also be between about 20 and 40, instead of being The present invention is also not restricted to the pressure compensating devices with a so-called closed cycle described above merely by way of example. It is possible, for example, to place the pressure chamber provided at the lower end of the cylinder in communication with the internal space of the tubular push rod following at the bottom by way of an axial bore and supply the pressure chamber with pressure oil through the push rod from the lubricating oil circulation of an internal combustion engine. In this case, the push rod forms the reservoir, and the bore leading to the pressure chamber represents the passage, which is provided with the above-described relief valve on the side of the pressure chamber. In this connection, the bottom of the piston of the compensating mechanism is closed. The oil leakage containing the air bubbles can return to the oil sump of the internal combustion engine from the annular gap between the piston and the cylinder, for example, directly via appropriate drainage channels.

While, therefore, I have shown and described several embodiments in accordance with the present invention, it is to be understood that the same is susceptible to numerous changes and modifications as known to one skilled in the art, and I do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the present invention.

I claim:

1. A clearance compensatingmechanism for reciprocating drive linkages, especially for valve drives of internal combustion engines, comprising a cylinder; a piston movably mounted in said cylinder; a pressure chamber adapted to be filled with a liquid medium disposed immediately ahead of said piston; said piston and said cylinder defining a gap therebetween for permitting a minor amount of said medium to leak therethrough; reservoir means adapted to be filled at least partially with said medium; passage means including relief valve means disposed below an inlet to said gap for connecting said reservoir to said pressure chamber; and biasing means for biasing said piston in an outwardly direction with respect to said cylinder; characterized in that the pressure chamber exhibits a continuously increasing throughflow crosssection in an approximately funnel-like manner from the inlet to the gap downward to the passage means and in that the walls of said pressure chamber immediately adjacent the inlet to the gap form a maximum angle of approximately 20 with respect to one another, wherein said biasing means comprises a compression spring supported at a spacing from a wall surface of said piston on pin means extending approximately radially outward from said wall surface.

2. A mechanism according to claim 1, wherein said relief valve means includes a valve body and cage means for holding said valve body.

3. A clearance compensating mechanism for reciprocating drive linkages, especiallyfor valve drives of internal combustion engines, comprising a cylinder; a piston movably mounted in said cylinder; a pressure chamber adapted to be filled with a liquid medium disposed immediately ahead of said piston; said piston and said cylinder defining a gap therebetween for permitting a minor amount of said medium to leak therethrough; reservoir means adapted to be filled at least partially with said medium; passage means including relief valve means disposed below an inlet to said gap for connecting said reservoir to said pressure chamber; and biasing means for biasing said piston in an outwardly direction with respect to said cylinder; characterized in that the pressure chamber exhibits a continuously increasing throughflow crosssection in an approximately funnel-like manner from the inlet to the gap downward to the passage means and in that the walls of said pressure chamber immediately adjacent the inlet to the gap form a maximum angle of approximately 20 with respect to one another, characterized in that the cylinder has a substantially constant circular cross-section along its axial direction and in that said piston extends downwardly into said cylinder, the surface of said piston having an uppermost portion of substanfially circular cylinder shape which merges into an intermediate portion shaped as a frustrum of a first downwardly pointing cone having a conical angle of between 20 and 40 and said intermediate portion merges into a lower most portion shaped as a second downwardly pointing cone having a conical angle of between and I20".

4. A mechanism according to claim 3, characterized in that the conical angle of said first downwardly pointing cone is approximately 30.

5. A mechanism according to claim 3, characterized in that said relief valve comprises a check valve having a movable valve body and a cage for holding the valve body, said cage including a narrow leaf spring extending radially under the valve body.

6. A mechanism according to claim 3, characterized in that the biasing means includes a compression spring positioned between the bottom of the inside of the cylinder and the piston, said spring being supported at the piston by pins extending radially outwardly from the lowermost portion of the piston.

7. A mechanism according to claim 3, characterized in that the difference between the maximum and minimum radii of the frustrum of the cone forming the intermediate portion of said piston is approximately lmm.

8. A clearance compensating mechanism for reciprocating drive linkages, especially for valve drives of internal combustion engines, comprising a cylinder; a piston movably mounted in said cylinder; a pressure chamber adapted to be filled with a liquid medium disposed immediately ahead of said piston, said piston and said cylinder defining a gap therebetween for permitting a minor amount of said medium to leak therethrough; reservoir means adapted to be filled at least partially with said medium; passage means including relief valve means disposed below an inlet to said gap for connecting said reservoir to said pressure chamber; and biasing means for biasing said piston in an outwardly direction with respect to said cylinder; characterized in that the pressure chamber exhibits a continuously increasing throughflow crosssection in an approximately funnellike manner from the inlet to the gap downward to the passage means and in that the walls of said pressure chamber immediately adjacent the inlet to the gap form a maximum angle of approximately 20 with respect to one another, characterized in that there is a maximum distance of approximately 1mm between the pressure chamber walls in the area immediately adjacent the inlet to the gap where the maximum angle of 20 between said walls exists. 

1. A clearance compensating mechanism for reciprocating drive linkages, especially for valve drives of internal combustion engines, comprising a cylinder; a piston movably mounted in said cylinder; a pressure chamber adapted to be filled with a liquid medium disposed immediately ahead of said piston; said piston and said cylinder defining a gap therebetween for permitting a minor amount of said medium to leak therethrough; reservoir means adapted to be filled at least partially with said medium; passage means including relief valve means disposed below an inlet to said gap for connecting said reservoir to said pressure chamber; and biasing means for biasing said piston in an outwardly direction with respect to said cylinder; characterized in that the pressure chamber exhibits a continuously increasing throughflow cross-section in an approximately funnel-like manner from the inlet to the gap downward to the passage means and in that the walls of said pressure chamber immediately adjacent the inlet to the gap form a maximum angle of approximately 20* with respect to one another, wherein said biasing means comprises a compression spring supported at a spacing from a wall surface of said piston on pin means extending approximately radially outward from said wall surface.
 2. A mechanism according to claim 1, wherein said relief valve means includes a valve body and cage means for holding said valve body.
 3. A clearance compensating mechanism for reciprocating drive linkages, especially for valve drives of internal combustion engines, comprising a cylinder; a piston movably mounted in said cylinder; a pressure chamber adapted to be filled with a liquid medium disposed immediately ahead of said piston; said piston and said cylinder defining a gap therebetween for permitting a minor amount of said medium to leak therethrough; reservoir means adapted to be filled at least partially with said medium; passage means including relief valve means disposed below an inlet to said gap for connecting said reservoir to said pressure chamber; and biasing means for biasing said piston in an outwardly direction with respect to said cylinder; characterized in that the pressure chamber exhibits a continuously increasing throughflow cross-section in an approximately funnel-like manner from the inlet to the gap downward to the passage means and in that the walls of said pressure chamber immediately adjacent the inlet to the gap form a maximum angle of approximately 20* with respect to one another, characterized in that the cylinder has a substantially constant circular cross-section along its axial direction and in that said piston extends downwardly into said cylinder, the surface of said piston having an uppermost portion of substantially circular cylinder shape which merges into an intermediate portion shaped as a frustrum of a first downwardly pointing cone having a conical angle of between 20* and 40* and said intermediate portion merges into a lowermost portion shaped as a second downwardly pointing cone having a conical angle of between 90* and 120*.
 4. A mechaniSm according to claim 3, characterized in that the conical angle of said first downwardly pointing cone is approximately 30*.
 5. A mechanism according to claim 3, characterized in that said relief valve comprises a check valve having a movable valve body and a cage for holding the valve body, said cage including a narrow leaf spring extending radially under the valve body.
 6. A mechanism according to claim 3, characterized in that the biasing means includes a compression spring positioned between the bottom of the inside of the cylinder and the piston, said spring being supported at the piston by pins extending radially outwardly from the lowermost portion of the piston.
 7. A mechanism according to claim 3, characterized in that the difference between the maximum and minimum radii of the frustrum of the cone forming the intermediate portion of said piston is approximately 1mm.
 8. A clearance compensating mechanism for reciprocating drive linkages, especially for valve drives of internal combustion engines, comprising a cylinder; a piston movably mounted in said cylinder; a pressure chamber adapted to be filled with a liquid medium disposed immediately ahead of said piston, said piston and said cylinder defining a gap therebetween for permitting a minor amount of said medium to leak therethrough; reservoir means adapted to be filled at least partially with said medium; passage means including relief valve means disposed below an inlet to said gap for connecting said reservoir to said pressure chamber; and biasing means for biasing said piston in an outwardly direction with respect to said cylinder; characterized in that the pressure chamber exhibits a continuously increasing throughflow cross-section in an approximately funnel-like manner from the inlet to the gap downward to the passage means and in that the walls of said pressure chamber immediately adjacent the inlet to the gap form a maximum angle of approximately 20* with respect to one another, characterized in that there is a maximum distance of approximately 1mm between the pressure chamber walls in the area immediately adjacent the inlet to the gap where the maximum angle of 20* between said walls exists. 